1. Field of the Invention
The present invention relates to a system for detecting an abnormal state of a power supply and, in particular, to a system for detecting an abnormal state of a power supply, in an externally discernible fashion, which can be applied to an automobile's stereo set or the like with a battery, in particular, used as a power supply.
2. Description of the Related Art
Generally, an overvoltage protection circuit is built in a low-frequency power amplifier so as to protect the latter from being destroyed by a power supply's surge or overvoltage or from being degenerated in its characteristic. In the case where a stereo set is used as an attachment on automobiles in particular, such a protection device is essential to a recent low-frequency amplifier because there is a high possibility that overvoltage or various surges will be supplied to that power supply.
FIG. 1 shows a conventional overvoltage protection circuit as one form of a monolithic IC element on an automobile.
In FIG. 1, a resistor R1, zener diodes Z1, Z2, Z3 and Z4 and resistor R2 are series connected across a power supply V.sub.CC line and a ground GND line. Resistors R3 and R4 are connected at one end to the power supply V.sub.CC and at the other end to the emitters of PNP transistors Q1 and Q2. The transistor Q1 has its base connected to the base of the transistor Q2 and to its own collector to function as a diode. NPN transistor Q3 has its collector connected to the collector of the transistor Q1 and its base to a junction of the zener diode Z4 and resistor R2 and its emitter connected to the ground GND line. A bias circuit 10 is connected through a resistor R5 to the collector of the transistor Q2.
In the overvoltage protection circuit thus arranged, a voltage on the power supply line is normally monitored by stack voltages (V.sub.Z1, V.sub.Z2, V.sub.Z3 and V.sub.Z4) corresponding to zener voltages V.sub.Z1, V.sub.Z2, V.sub.Z3 and V.sub.Z4 on the zener diodes Z1, Z2, Z3 and Z4. If any overvoltage or surge satisfying the following condition "Power Supply Voltage: V.sub.CC &gt;(V.sub.Z1 +V.sub.Z2 +V.sub.Z3 +V.sub.Z4)" is applied to the power supply line, then the NPN transistor Q3 is turned ON and the PNP transistor Q constituting part of a constant current mirror circuit is turned ON, causing current to flow through the resistor R5 so that the bias circuit is blocked. By doing so, it is possible to prevent a low-frequency power amplifier, such as an audio power IC element, from being destroyed or degenerated in its characteristic.
However, the overvoltage protection circuit cannot protect the low-frequency amplifier from all the surge applied to the power supply line. That is, the surge, upon being applied to the associated circuit to a level exceeding a net level of the overvoltage protection circuit, will cause the destruction of the low-frequency amplifier or the degeneration of its characteristic.
In order to increase the net value of the overvoltage protection circuit, it will be necessary to raise the withstand voltage of, for example, the aforementioned audio power IC element. To this end, the IC chip size becomes greater, resulting in a high cost.
If a power IC element on the market is being degenerated in its characteristic or destroyed as already set forth above, it will be difficult to evaluate its real cause or its real adverse situations. That is, it is not possible to externally evaluate any overvoltage on the power line and an associated circuit's abnormal or off-normal state caused by the application of an abnormal voltage. It is, therefore, not possible to clearly solve such a problem as caused by a low-frequency amplifier in a continuous abnormal state, such as a defective power IC element sometimes encountered as an off-the-shelf product, and it becomes very difficult to find out that cause of such abnormality on the associated circuit or element.